System and method for managing refractory periods in a cardiac rhythm management device with biventricular sensing

ABSTRACT

A method and system for managing refractory periods in a cardiac rhythm management device configured for biventricular or biatrial sensing. Refractory periods for each channel of the pacemaker are provided by interval timers that are triggered by sensed or paced events in order to prevent misinterpretation of sensing signals.

FIELD OF THE INVENTION

[0001] This invention pertains to methods and systems for operating acardiac rhythm management device. In particular, the invention relatesto defining and managing periods during which sensing channels of thedevice are rendered refractory.

BACKGROUND

[0002] Cardiac rhythm management devices are implantable devices thatprovide electrical stimulation to selected chambers of the heart inorder to treat disorders of cardiac rhythm. A pacemaker, for example, isa cardiac rhythm management device that paces the heart with timedpacing pulses. The most common condition for which pacemakers are usedis in the treatment of bradycardia, where the ventricular rate is tooslow. Atrio-ventricular conduction defects (i.e., AV block) that arefixed or intermittent and sick sinus syndrome represent the most commoncauses of bradycardia for which permanent pacing may be indicated. Iffunctioning properly, the pacemaker makes up for the heart's inabilityto pace itself at an appropriate rhythm in order to meet metabolicdemand by enforcing a minimum heart rate. Pacing therapy may also beapplied in order to treat cardiac rhythms that are too fast, termedanti-tachycardia pacing. (As the term is used herein, a pacemaker is anycardiac rhythm management device with a pacing functionality, regardlessof any other functions it may perform such as cardioversion ordefibrillation.)

[0003] Also included within the concept of cardiac rhythm is the degreeto which the heart chambers contract in a coordinated manner during acardiac cycle to result in the efficient pumping of blood. The heart hasspecialized conduction pathways in both the atria and the ventriclesthat enable the rapid conduction of excitation (i.e., depolarization)throughout the myocardium. These pathways conduct excitatory impulsesfrom the sino-atrial node to the atrial myocardium, to theatrio-ventricular node, and thence to the ventricular myocardium toresult in a coordinated contraction of both atria and both ventricles.This both synchronizes the contractions of the muscle fibers of eachchamber and synchronizes the contraction of each atrium or ventriclewith the contralateral atrium or ventricle. Without the synchronizationafforded by the normally functioning specialized conduction pathways,the heart's pumping efficiency is greatly diminished. Patients whoexhibit pathology of these conduction pathways, such as bundle branchblocks, can thus suffer compromised cardiac output.

[0004] Patients with conventional pacemakers can also have compromisedcardiac output because artificial pacing with an electrode fixed into anarea of the myocardium does not take advantage of the above-describedspecialized conduction system. This is because the specializedconduction system can only be entered by impulses emanating from thesino-atrial or atrio-ventricular nodes. The spread of excitation from asingle pacing site must proceed only via the much slower conductingmuscle fibers of either the atria or the ventricles, resulting in thepart of the myocardium stimulated by the pacing electrode contractingwell before parts of the chamber located more distally to the electrode,including the myocardium of the chamber contralateral to the pacingsite. Although the pumping efficiency of the heart is somewhat reducedfrom the optimum, most patients can still maintain more than adequatecardiac output with artificial pacing.

[0005] Heart failure is clinical syndrome in which an abnormality ofcardiac function causes cardiac output to fall below a level adequate tomeet the metabolic demand of peripheral tissues and is usually referredto as congestive heart failure (CHF) due to the accompanying venous andpulmonary congestion. CHF can be due to a variety of etiologies withischemic heart disease being the most common. Some CHF patients sufferfrom some degree of AV block or are chronotropically deficient such thattheir cardiac output can be improved with conventional bradycardiapacing. Such pacing, however, may result in some degree ofuncoordination in atrial and/or ventricular contractions due to the wayin which pacing excitation is spread throughout the myocardium asdescribed above. The resulting diminishment in cardiac output may besignificant in a CHF patient whose cardiac output is alreadycompromised. Intraventricular and/or interventricular conduction defects(e.g., bundle branch blocks) are also commonly found in CHF patients. Inorder to treat these problems, cardiac rhythm management devices havebeen developed which provide pacing stimulation to one or more heartchambers in an attempt to improve the coordination of atrial and/orventricular contractions, termed cardiac resynchronization therapy.

[0006] In conventional pacemakers with sensing channels for sensing oneor more heart chambers, the ventricular and/or atrial sensing channelsare rendered refractory following certain events, such that certainsensed events are ignored for the duration of the period. Sensingchannels are rendered refractory both in order to prevent reentry intothe system of an output pacing pulse (in which case the sensingamplifiers are blanked) and to prevent the misinterpretation of inputdata by the sensing of afterpotentials or by crosstalk between sensingchannels. Cardiac resynchronization therapy may involve pacing bothatria, both ventricles, or a heart chamber at one or more pacing sitesbased upon senses from another site. In order to control the pacing andavoid pacing a chamber or site in the presence of intrinsic activity,sensing channels should be provided for each chamber or site. Sensingboth ventricles or both atria, however, requires that refractory periodsbe managed differently from the situation where only one atria or oneventricle is sensed.

SUMMARY OF THE INVENTION

[0007] The present invention is a system and method for managing therefractory periods of sensing channels in a cardiac rhythm managementdevice in which both ventricles, both atria, and/or multiple sites inthe same heart chamber are sensed. Such sensing configurations can mostusefully be employed in delivering cardiac resychronization theapy. Therefractory periods for the sensing channels are managed in a manner suchthat misinterpretation of sense signals is avoided while still allowingthe device to pace one or more chambers safely and at the appropriatetimes.

[0008] In an exemplary cardiac resynchronization pacing configuration,heart chambers designated as a rate chamber and a synchronized chamberare sensed through separate channels, and at least one chamber is pacedupon expiration of an escape interval without receipt of a rate chambersense signal. In accordance with the invention, each sensing channel isrendered refractory for a separately selected pacing refractory periodafter a pacing event, and a sensing channel is rendered refractory for aselected sensing refractory period after receipt of a sense signal fromthat channel while leaving the other channel non-refractory.

[0009] In a particular embodiment of the invention, the rate andsynchronized chambers are the right and left ventricles, respectively,and an atrial sensing and pacing channel is also present. The atrialsensing channel may be rendered refractory for selected intervalsfollowing an atrial sense, an atrial pace, a pace of either ventricle,or a right ventricular sense signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a system diagram of a pacemaker configured forbiventricular pacing and sensing.

[0011]FIG. 2 shows refractory periods following a sensed atrial event.

[0012]FIG. 3 shows refractory periods following an atrial pace.

[0013]FIG. 4 shows refractory periods after a right ventricular sense.

[0014]FIG. 5 shows refractory periods after a left ventricular sense.

[0015]FIGS. 6 and 12 shows refractory periods following a ventricularpace in biventricular pacing mode with a positive biventricular delay.

[0016]FIG. 7 shows refractory periods following a ventricular pace inbiventricular pacing mode with a positive biventricular delay where theleft ventricular pace is inhibited.

[0017]FIGS. 8 and 11 shows refractory periods periods following aventricular pace in biventricular pacing mode with a negativebiventricular delay.

[0018]FIG. 9 shows refractory periods following a ventricular pace inbiventricular pacing mode with a negative biventricular delay where theleft ventricular pace is inhibited.

[0019]FIG. 10 shows refractory periods following a ventricular pace inbiventricular pacing mode with simultaneous ventricular pacing.

[0020]FIG. 13 shows refractory periods following a ventricular pace in aright ventricle-only pacing mode.

[0021]FIG. 14 shows refractory periods following a ventricular pace in aleft ventricle-only pacing mode.

[0022]FIG. 15 shows refractory periods following a right ventricularsafety pace.

[0023]FIG. 16 shows refractory periods for ventricular triggeredbiventricular pacing with inhibition of the left ventricular pace.

[0024]FIG. 17 shows refractory periods for ventricular triggeredbiventricular pacing where both ventricles are paced.

[0025]FIG. 18 shows refractory periods for ventricular triggered leftventricle-only pacing.

DESCRIPTION OF THE INVENTION

[0026] The present invention relates to a method and system for managingrefractory periods for multiple sensing channels in order to safely andeffectively implement a synchronized pacing mode. As will be describedbelow, such modes may be used to deliver cardiac resynchronizationtherapy.

[0027] 1. Hardware Platform

[0028] Pacemakers are typically implanted subcutaneously orsubmuscularly and have leads threaded intravenously into the heart toconnect the device to electrodes used for sensing and pacing. Aprogrammable electronic controller causes the pacing pulses to be outputin response to lapsed time intervals and sensed electrical activity(i.e., intrinsic heart beats not as a result of a pacing pulse).Pacemakers sense intrinsic cardiac electrical activity by means ofinternal electrodes disposed within or near the chamber to be sensed. Adepolarization wave associated with an intrinsic contraction of theatria or ventricles that is detected by the pacemaker is referred to asan atrial sense or ventricular sense, respectively. In order to causesuch a contraction in the absence of an intrinsic beat, a pacing pulse(either an atrial pace or a ventricular pace) with energy above acertain pacing threshold is delivered to the chamber.

[0029]FIG. 1 shows a system diagram of a microprocessor-based pacemakerphysically configured with sensing and pacing channels for both atriaand both ventricles. The controller 10 of the pacemaker is amicroprocessor which communicates with a memory 12 via a bidirectionaldata bus. The memory 12 typically comprises a ROM (read-only memory) forprogram storage and a RAM (random-access memory) for data storage. Thepacemaker has atrial sensing and pacing channels comprising electrode 34a-b, leads 33 a-b, sensing amplifiers 31 a-b, pulse generators 32 a-b,and atrial channel interfaces 30 a-b which communicate bidirectionallywith microprocessor 10. The device also has ventricular sensing andpacing channels for both ventricles comprising electrodes 24 a-b, leads23 a-b, sensing amplifiers 21 a-b, pulse generators 22 a-b, andventricular channel interfaces 20 a-b. In the figure, “a” designates oneventricular or atrial channel and “b” designates the channel for thecontralateral chamber. In this embodiment, a single electrode is usedfor sensing and pacing in each channel, known as a unipolar lead. Otherembodiments may employ bipolar leads which include two electrodes foroutputting a pacing pulse and/or sensing intrinsic activity. The channelinterfaces 20 a-b and 30 a-b include analog-to-digital converters fordigitizing sensing signal inputs from the sensing amplifiers andregisters which can be written to by the microprocessor in order tooutput pacing pulses, change the pacing pulse amplitude, and adjust thegain and threshold values for the sensing amplifiers. An exertion levelsensor 330 (e.g., an accelerometer or a minute ventilation sensor)enables the controller to adapt the pacing rate in accordance withchanges in the patient's physical activity. A telemetry interface 40 isalso provided for communicating with an external programmer 500 whichhas an associated display 510. A pacemaker incorporating the presentinvention may possess all of the components in FIG. 1 and beprogrammable so as to operate in a number of different modes, or it mayhave only those components necessary to operate in a particular mode.

[0030] The controller 10 controls the overall operation of the device inaccordance with programmed instructions stored in memory. The controller10 controls the delivery of paces via the pacing channels, interpretssense signals from the sensing channels, implements timers for definingescape intervals and sensory refractory periods, and performs the pacecounting functions as described below. It should be appreciated,however, that these functions could also be performed by custom logiccircuitry either in addition to or instead of a programmedmicroprocessor.

[0031] 2. Bradycardia Pacing Modes

[0032] Bradycardia pacing modes refer to pacing algorithms used to pacethe atria and/or ventricles when the intrinsic ventricular rate isinadequate either due to AV conduction blocks or sinus node dysfunction.Such modes may either be single-chamber pacing, where either an atriumor a ventricle is paced, or dual-chamber pacing in which both an atriumand a ventricle are paced. The modes are generally designated by aletter code of three positions where each letter in the code refers to aspecific function of the pacemaker. The first letter refers to whichheart chambers are paced and which may be an A (for atrium), a V (forventricle), D (for both chambers), or O (for none). The second letterrefers to which chambers are sensed by the pacemaker's sensing channelsand uses the same letter designations as used for pacing. The thirdletter refers to the pacemaker's response to a sensed P wave from theatrium or an R wave from the ventricle and may be an I (for inhibited),T (for triggered), D (for dual in which both triggering and inhibitionare used), and O (for no response). Modem pacemakers are typicallyprogrammable so that they can operate in any mode which the physicalconfiguration of the device will allow. Additional sensing ofphysiological data allows some pacemakers to change the rate at whichthey pace the heart in accordance with some parameter correlated tometabolic demand. Such pacemakers are called rate-adaptive pacemakersand are designated by a fourth letter added to the three-letter code, R.

[0033] Pacemakers can enforce a minimum heart rate either asynchronouslyor synchronously. In asynchronous pacing, the heart is paced at a fixedrate irrespective of intrinsic cardiac activity. There is thus a riskwith asynchronous pacing that a pacing pulse will be deliveredcoincident with an intrinsic beat and during the heart's vulnerableperiod which may cause fibrillation. Most pacemakers for treatingbradycardia today are therefore programmed to operate synchronously in aso-called demand mode where sensed cardiac events occurring within adefined interval either trigger or inhibit a pacing pulse. Inhibiteddemand pacing modes utilize escape intervals to control pacing inaccordance with sensed intrinsic activity. In an inhibited demand mode,a pacing pulse is delivered to a heart chamber during a cardiac cycleonly after expiration of a defined escape interval during which nointrinsic beat by the chamber is detected. If an intrinsic beat occursduring this interval, the heart is thus allowed to “escape” from pacingby the pacemaker. Such an escape interval can be defined for each pacedchamber. For example, a ventricular escape interval can be definedbetween ventricular events so as to be restarted with each ventricularsense or pace. The inverse of this escape interval is the minimum rateat which the pacemaker will allow the ventricles to beat, sometimesreferred to as the lower rate limit (LRL).

[0034] In atrial tracking pacemakers (i.e., VDD or DDD mode), anotherventricular escape interval is defined between atrial and ventricularevents, referred to as the atrio-ventricular interval (AVI). Theatrio-ventricular interval is triggered by an atrial sense or pace andstopped by a ventricular sense or pace. A ventricular pace is deliveredupon expiration of the atrio-ventricular interval if no ventricularsense occurs before. Atrial-tracking ventricular pacing attempts tomaintain the atrio-ventricular synchrony occurring with physiologicalbeats whereby atrial contractions augment diastolic filling of theventricles. If a patient has a physiologically normal atrial rhythm,atrial-tracking pacing also allows the ventricular pacing rate to beresponsive to the metabolic needs of the body.

[0035] A pacemaker can also be configured to pace the atria on aninhibited demand basis. An atrial escape interval is then defined as themaximum time interval in which an atrial sense must be detected after aventricular sense or pace before an atrial pace will be delivered. Whenatrial inhibited demand pacing is combined with atrial-triggeredventricular demand pacing (i.e., DDD mode), the lower rate limitinterval is then the sum of the atrial escape interval and theatrio-ventricular interval.

[0036] Another type of synchronous pacing is atrial-triggered orventricular-triggered pacing. In this mode, an atrium or ventricle ispaced immediately after an intrinsic beat is detected in the respectivechamber. Triggered pacing of a heart chamber is normally combined withinhibited demand pacing so that a pace is also delivered upon expirationof an escape interval in which no intrinsic beat occurs. Such triggeredpacing may be employed as a safer alternative to asynchronous pacingwhen, due to far-field sensing of electromagnetic interference fromexternal sources or skeletal muscle, false inhibition of pacing pulsesis a problem. If a sense in the chamber's sensing channel is an actualdepolarization and not a far-field sense, the triggered pace isdelivered during the chamber's physiological refractory period and is ofno consequence.

[0037] 3. Cardiac Resynchronization Therapy

[0038] Cardiac resynchronization therapy is pacing stimulation appliedto one or more heart chambers in a manner that restores or maintainssynchronized bilateral contractions of the atria and/or ventricles andthereby improves pumping efficiency. Certain patients with conductionabnormalities may experience improved cardiac synchronization withconventional single-chamber or dual-chamber pacing as described above.For example, a patient with left bundle branch block may have a morecoordinated contraction of the ventricles with a pace than as a resultof an intrinsic contraction. In that sense, conventional bradycardiapacing of an atrium and/or a ventricle may be considered asresynchronization therapy. Resynchronization pacing, however, may alsoinvolve pacing both ventricles and/or both atria in accordance with asynchronized pacing mode as described below. A single chamber may alsobe resynchronized to compensate for intra-atrial or intra-ventricularconduction delays by delivering paces to multiple sites of the chamber.

[0039] It is advantageous to deliver resynchronization therapy inconjunction with one or more synchronous bradycardia pacing modes, suchas are described above. One atrial and/or one ventricular pacing sitesare designated as rate sites, and paces are delivered to the rate sitesbased upon pacing and sensed intrinsic activity at the site inaccordance with the bradycardia pacing mode. In a single-chamberbradycardia pacing mode, for example, one of the paired atria or one ofthe ventricles is designated as the rate chamber. In a dual-chamberbradycardia pacing mode, either the right or left atrium is selected asthe atrial rate chamber and either the right or left ventricle isselected as the ventricular rate chamber. The heart rate and the escapeintervals for the pacing mode are defined by intervals between sensedand paced events in the rate chambers only. Resynchronization therapymay then be implemented by adding synchronized pacing to the bradycardiapacing mode where paces are delivered to one or more synchronized pacingsites in a defined time relation to one or more selected sensing andpacing events that either reset escape intervals or trigger paces in thebradycardia pacing mode. Multiple synchronized sites may be pacedthrough multiple synchronized sensing/pacing channels, and the multiplesynchronized sites may be in the same or different chambers as the ratesite.

[0040] In bilateral synchronized pacing, which may be either biatrial orbiventricular synchronized pacing, the heart chamber contralateral tothe rate chamber is designated as a synchronized chamber. For example,the right ventricle may be designated as the rate ventricle and the leftventricle designated as the synchronized ventricle, and the paired atriamay be similarly designated. Each synchronized chamber is then paced ina timed relation to a pace or sense occurring in the contralateral ratechamber.

[0041] One synchronized pacing mode may be termed offset synchronizedpacing. In this mode, the synchronized chamber is paced with a positive,negative, or zero timing offset as measured from a pace delivered to itspaired rate chamber, referred to as the synchronized chamber offsetinterval. The offset interval may be zero in order to pace both chamberssimultaneously, positive in order to pace the synchronized chamber afterthe rate chamber, or negative to pace the synchronized chamber beforethe rate chamber. One example of such pacing is biventricular offsetsynchronized pacing where both ventricles are paced with a specifiedoffset interval. The rate ventricle is paced in accordance with asynchronous bradycardia mode which may include atrial tracking, and theventricular escape interval is reset with either a pace or a sense inthe rate ventricle. (Resetting in this context refers to restarting theinterval in the case of an LRL ventricular escape interval and tostopping the interval in the case of an AVI.) Thus, a pair ofventricular paces are delivered after expiration of the AVI escapeinterval or expiration of the LRL escape interval, with ventricularpacing inhibited by a sense in the rate ventricle that restarts the LRLescape interval and stops the AVI escape interval. In this mode, thepumping efficiency of the heart will be increased in some patients bysimultaneous pacing of the ventricles with an offset of zero. However,it may be desirable in certain patients to pace one ventricle before theother in order to compensate for different conduction velocities in thetwo ventricles, and this may be accomplished by specifying a particularpositive or negative ventricular offset interval.

[0042] Another synchronized mode is triggered synchronized pacing. Inone type of triggered synchronized pacing, the synchronized chamber ispaced after a specified trigger interval following a sense in the ratechamber, while in another type the rate chamber is paced after aspecified trigger interval following a sense in the synchronizedchamber. The two types may also be employed simultaneously. For example,with a trigger interval of zero, pacing of one chamber is triggered tooccur in the shortest time possible after a sense in the other chamberin order to produce a coordinated contraction. (The shortest possibletime for the triggered pace is limited by a sense-to-pace latency perioddictated by the hardware.) This mode of pacing may be desirable when theintra-chamber conduction time is long enough that the pacemaker is ableto reliably insert a pace before depolarization from one chamber reachesthe other. Triggered synchronized pacing can also be combined withoffset synchronized pacing such that both chambers are paced with thespecified offset interval if no intrinsic activity is sensed in the ratechamber and a pace to the rate chamber is not otherwise delivered as aresult of a triggering event. A specific example of this mode isventricular triggered synchronized pacing where the rate andsynchronized chambers are the right and left ventricles, respectively,and a sense in the right ventricle triggers a pace to the left ventricleand/or a sense in the left ventricle triggers a pace to the rightventricle.

[0043] As with other synchronized pacing modes, the rate chamber in atriggered synchronized pacing mode can be paced with one or moresynchronous bradycardia pacing modes. If the rate chamber is controlledby a triggered bradycardia mode, a sense in the rate chamber sensingchannel, in addition to triggering a pace to the synchronized chamber,also triggers an immediate rate chamber pace and resets any rate chamberescape interval. The advantage of this modal combination is that thesensed event in the rate chamber sensing channel might actually be afar-field sense from the synchronized chamber, in which case the ratechamber pace should not be inhibited. In a specific example, the rightand left ventricles are the rate and synchronized chambers,respectively, and a sense in the right ventricle triggers a pace to theleft ventricle. If right ventricular triggered pacing is also employedas a bradycardia mode, both ventricles are paced after a rightventricular sense has been received to allow for the possibility thatthe right ventricular sense was actually a far-field sense of leftventricular depolarization in the right ventricular channel. If theright ventricular sense were actually from the right ventricle, theright ventricular pace would occur during the right ventricle'sphysiological refractory period and cause no harm.

[0044] As mentioned above, certain patients may experience some cardiacresynchronization from the pacing of only one ventricle and/or oneatrium with a conventional bradycardia pacing mode. It may be desirable,however, to pace a single atrium or ventricle in accordance with apacing mode based upon senses from the contralateral chamber. This mode,termed synchronized chamber-only pacing, involves pacing only thesynchronized chamber based upon senses from the rate chamber. One way toimplement synchronized chamber-only pacing is to pseudo-pace the ratechamber whenever the synchronized chamber is paced before the ratechamber is paced, such that the pseudo-pace inhibits a rate chamber paceand resets any rate chamber escape intervals. Such pseudo-pacing can becombined with the offset synchronized pacing mode using a negativeoffset to pace the synchronized chamber before the rate chamber and thuspseudo-pace the rate chamber, which inhibits the real scheduled ratechamber pace and resets the rate chamber pacing escape intervals. Oneadvantage of this combination is that sensed events in the rate chamberwill inhibit the synchronized chamber-only pacing, which may benefitsome patients by preventing pacing that competes with intrinsicactivation (i.e., fusion beats). Another advantage of this combinationis that rate chamber pacing can provide backup pacing when in asynchronized chamber-only pacing mode, such that when the synchronizedchamber pace is prevented, for example to avoid pacing during thechamber vulnerable period following a prior contraction, the ratechamber will not be pseudo-paced and thus will be paced upon expirationof the rate chamber escape interval. Synchronized chamber-only pacingcan be combined also with a triggered synchronized pacing mode, inparticular with the type in which the synchronized chamber is triggeredby a sense in the rate chamber. One advantage of this combination isthat sensed events in the rate chamber will trigger the synchronizedchamber-only pacing, which may benefit some patients by synchronizingthe paced chamber contractions with premature contralateral intrinsiccontractions.

[0045] An example of synchronized chamber-only pacing is leftventricle-only synchronized pacing where the rate and synchronizedchambers are the right and left ventricles, respectively. Leftventricle-only synchronized pacing may be advantageous where theconduction velocities within the ventricles are such that pacing onlythe left ventricle results in a more coordinated contraction by theventricles than with conventional right ventricular pacing orbiventricular pacing. Left ventricle-only synchronized pacing may beimplemented in inhibited demand modes with or without atrial tracking,similar to biventricular pacing. A left ventricular pace is thendelivered upon expiration of the AVI escape interval or expiration ofthe LRL escape interval, with left ventricular pacing inhibited by aright ventricular sense that restarts the LRL escape interval and stopsthe AVI escape interval.

[0046] In the synchronized modes described above, the rate chamber issynchronously paced with a mode based upon detected intrinsic activityin the rate chamber, thus protecting the rate chamber against pacesbeing delivered during the vulnerable period. In order to providesimilar protection to a synchronized chamber or synchronized pacingsite, a synchronized chamber protection period (SCPP) may be provided.(In the case of multi-site synchronized pacing, a similar synchronizedsite protection period may be provided for each synchronized site.) TheSCPP is a programmed interval which is initiated by sense or paceoccurring in the synchronized chamber during which paces to thesynchronized chamber are inhibited. For example, if the right ventricleis the rate chamber and the left ventricle is the synchronized chamber,a left ventricular protection period LVPP is triggered by a leftventricular sense which inhibits a left ventricular pace which wouldotherwise occur before the escape interval expires. The SCPP may beadjusted dynamically as a function of heart rate and may be differentdepending upon whether it was initiated by a sense or a pace. The SCPPprovides a means to inhibit pacing of the synchronized chamber when apace might be delivered during the vulnerable period or when it mightcompromise pumping efficiency by pacing the chamber too close to anintrinsic beat. In the case of a triggered mode where a synchronizedchamber sense triggers a pace to the synchronized chamber, the pacingmode may be programmed to ignore the SCPP during the triggered pace.Alternatively, the mode may be programmed such that the SCPP starts onlyafter a specified delay from the triggering event, which allowstriggered pacing but prevents pacing during the vulnerable period.

[0047] In the case of synchronized chamber-only synchronized pacing, asynchronized chamber pace may be inhibited if a synchronized chambersense occurs within a protection period prior to expiration of the ratechamber escape interval. Since the synchronized chamber pace isinhibited by the protection period, the rate chamber is not pseudo-pacedand, if no intrinsic activity is sensed in the rate chamber, it will bepaced upon expiration of the rate chamber escape intervals. The ratechamber pace in this situation may thus be termed a safety pace. Forexample, in left ventricle-only synchronized pacing, a right ventricularsafety pace is delivered if the left ventricular pace is inhibited bythe left ventricular protection period and no right ventricular sensehas occurred.

[0048] As noted above, synchronized pacing may be applied to multiplesites in the same or different chambers. The synchronized pacing modesdescribed above may be implemented in a multi-site configuration bydesignating one sensing/pacing channel as the rate channel forsensing/pacing a rate site, and designating the other sensing/pacingchannels in either the same or the contralateral chamber as synchronizedchannels for sensing/pacing one or more synchronized sites. Pacing andsensing in the rate channel then follows rate chamber timing rules,while pacing and sensing in the synchronized channels followssynchronized chamber timing rules as described above. The same ordifferent synchronized pacing modes may be used in each synchronizedchannel.

[0049] 4. Refractory Period Management

[0050] The present invention is a system and method for managingrefractory periods. A refractory period is an interval during whichsensed activity from a sensing channel neither inhibits nor triggers apacing pulse. However, sensed events during the refractory period areoften used for other device algorithms such as those used to detectatrial arrhythmias and those used to extend the refractory period in thepresence of persistent electromagnetic interference.

[0051] Refractory periods are often implemented using a blankinginterval. When used, the blanking interval usually constitutes the firstpart of the refractory period. During the blanking interval, the deviceignores all electrical activity sensed by the sensing channel. Blankingintervals can shield the sensing channel from pacing artifacts orcross-chamber depolarization effects. Refractory periods are also oftenimplemented using a retriggerable noise interval, where the noiseinterval often constitutes the last part of the refractory period.Sensed events occurring within the noise interval will restart the noiseinterval, thus increasing the length of the refractory period.

[0052] The following description of the invention dealing withrefractory period management is set forth with respect to an exemplarycardiac rhythm management device that provides ventricular pacing in asynchronized mode and in which the rate and synchronized chambers arethe right and left ventricles, respectively. It should be appreciated,however, that similar embodiments could be constructed in accordancewith the description by replacing the right and left ventricles withother heart chambers or cardiac sites designated as rate andsynchronized chambers.

[0053] The exemplary device provides sensing/pacing channels for anatrium and both ventricles. FIGS. 2-18 depict cardiac events andillustrate the refractory periods for the sensing channels. Suchrefractory periods may be provided by timers implemented in hardware orsoftware executed by the controller in FIG. 1. Each refractory period isdefined by its sensing channel and the particular cardiac or pacemakerevent that triggers it. Refractory periods following sensed (i.e.,intrinsic) or paced events will be referred to as sensing or pacingrefractory periods, respectively.

[0054]FIG. 2 shows the refractory periods of the atrial and both rightand left ventricular channels after a sensed atrial event. The atrialchannel is rendered refractory for a selected interval ARPAS whichprevents any electrical events occurring after the atrial depolarizationand before expiration of the interval from being interpreted as anatrial sense. The right ventricular channel continues normal sensing aswith conventional pacemakers since far-field sensing of the atrium by aright ventricular electrode is unlikely. A refractory period for theleft ventricular channel may be necessary, however. Since a leftventricular electrode is normally placed via the coronary sinus, it maybe placed rather high in ventricle in close proximity to the leftatrium. In order to prevent oversensing of the left atrium by the leftventricular channel, the channel can be made refractory for a selectedinterval LVRPAS.

[0055]FIG. 3 depicts the refractory periods following an atrial pace.The atrial, right ventricular, and left ventricular channels arerendered refractory for selected intervals ARPAP, RVRPAP, and LVRPAP,respectively. Note that although RVRPAP and LVRPAP are shown as beingequal, it may be advantageous to extend the left ventricular refractoryperiod beyond the right ventricular refractory period in order to allowfor the added time that a depolarization from a right atrial pace takesto reach the left atrium where it could possibly be oversensed by theleft ventricular channel. There may also be instances where it would beadvantageous to set the RVRPAP longer than the LVRPAP.

[0056]FIG. 4 shows the sensing refractory periods after a sensedintrinsic right ventricular depolarization. The atrial and rightventricular channels are rendered refractory for separately selectedintervals ARPRVS (which is commonly referred to as the post-ventricularatrial refractory period or PVARP, used primarily to prevent pacemakermediated tachycardia) and RVRPRVS as is done for a conventionaldual-chamber pacemaker with sensing only in the right ventricle. Theleft ventricular sensing channel remains non-refractory, however, inorder to continue sensing for a depolarization which would inhibit aleft ventricular pace output and to allow diagnostic functions in thecardiac rhythm management device to count the intrinsic left ventricularevent. For left ventricular pacing in which the lower rate limitinterval is controlled by right ventricular events (i.e., where pacingof either ventricle is inhibited by a right ventricular sense), it isdesirable to provide a time interval triggered by a left ventricularsense signal during which pacing of the left ventricle is inhibited.That inhibitory interval, referred to herein as LVPP for leftventricular protective period, serves to prevent a left ventricular paceduring the vulnerable period following a depolarization which in somepatients can trigger ventricular arrhythmias.

[0057]FIG. 5 shows the refractory periods of the sensing channelsfollowing an intrinsic left ventricular event. Only the left ventriclechannel is rendered refractory for a selected interval LVRPLVS. In ordernot to impact pacing algorithms dependent upon right ventricular andatrial events, it is advantageous to not initiate refractory periods ineither of these sensing channels. Note that an LVPP interval may also betriggered by the left ventricular sense.

[0058]FIGS. 6 through 10 show the refractory periods of the sensingchannels after ventricular pace events when the pacemaker is operated inbiventricular pace mode with no ventricular triggering. In this mode,ventricular pacing is triggered by an atrial sense after expiration ofthe AVI or expiration of the LRL interval. In either case, ventricularpacing is inhibited by a right ventricular sense. A left ventricularpace is output offset from a right ventricle pace by a biventricularoffset interval BVD (i.e., a synchronized chamber offset interval). Theinterval BVD may be zero in order to pace both ventriclessimultaneously, positive in order to pace the left ventricle after theright, or negative if the left ventricle is paced before the right.

[0059]FIG. 6 shows the pacing refractory periods defined by intervalsARPVP, RVRPVP, and LVRPVP for the atrial, right ventricular, and leftventricular channels, respectively, after a biventricular pace with apositive BVD. Intervals ARPVP, RVRPVP and LVRPVP start at the instant ofthe right ventricular pace. It should be noted that, in order to preventsensing the depolarization produced by the pace, the left ventricularchannel should be rendered refractory following a right ventricle pacefor a period of time long enough to encompass the time it takes for adepolarization wave to reach the left ventricle. The right to leftventricle conduction time may be short enough so that no additionalrefractory time is needed following the left ventricular pace whichoccurs after the delay interval BVD. However, it may be advantageous tomaintain a minimum interval MINREF from the left ventricular pace to theend of the RVRPVP and LVRPVP. This means that pacing refractoryintervals for the atrial and both ventricular sensing channels inbiventricular pacing mode can be defined so as to be triggered bywhichever ventricular pace occurs first. Thus both RVRPVP and LVRPVP canbe triggered by the right ventricular pace when BVD is positive. FIG. 8shows the refractory periods for biventricular pacing mode in which theleft ventricle is paced first followed by a right ventricular pace aftera selected biventricular delay interval BVD (i.e., BVD is negative). Therefractory period intervals ARPVP, RVRPVP, and LVRPVP for the atrial andventricular channels are similar to those of FIG. 6 except that they aretriggered by a left ventricular pace. FIG. 10 shows the refractoryperiods in biventricular pacing mode with the ventricles pacedsimultaneously (i.e., when BVD is zero) again showing the intervalsARPVP, RVRPVP, and LVRPVP triggered by either of the ventricular paces.

[0060] In another embodiment, during biventricular pacing with anon-zero BVD, the refractory periods for the atrium and ventricles arestarted on the last ventricular pace. This is depicted in FIGS. 11 and12. As shown, refractory periods ARPVP1, RVRPVP1, and LVRPVP1 aretriggered by the leading ventricular pace. The lagging ventricular pacetriggers refractory periods ARPVP2, RVRPVP2, and LVRPVP2. If any of theARPVP1, RVRPVP1, or LVRPVP1 periods exceed the BVD, that refractoryperiod is truncated at the time of the lagging ventricular pace. TheARPVP1, RVRPVP1, and LVRPVP1 periods may be the same or different as theARPVP2, RVRPVP2, and LVRPVP2 periods.

[0061] In any of the non-ventricular-triggered biventricular pacingmodes, the left ventricular pace is inhibited if the pace would fallwithin the left ventricular protective period interval LVPP. The LVPPmay be triggered by a left ventricular sense occurring before the rightventricular pace (i.e., before the onset of the left ventricularrefractory period). In that event, as shown in FIG. 7 with respect to abiventricular pacing mode with a positive BVD, the refractory intervalsare triggered by the right ventricular pace and are left unchanged fromthose of FIG. 6. The event labeled as a left ventricular pseudo-pacerepresents where the left ventricular pace would have occurred were itnot inhibited. FIG. 9 shows that the refractory intervals are triggeredby the right ventricular pace when the right ventricular pace is tofollow the left ventricular pace (i.e., BVD is negative) but no leftventricular pace occurs due to triggering of the LVPP. The pacingrefractory periods with simultaneous ventricular pacing are similarlyunchanged when left ventricular pacing is inhibited. Thus the earliestventricular pace output occurring can be used to trigger the pacingrefractory periods regardless of the value of the biventricular delayand whether or not the left ventricular protective period is triggered.In another embodiment, the refractory periods are triggered by the rightventricular pace but set to a value different than that used duringbiventricular pacing.

[0062] In some situations, the best hemodynamic effect may result frompacing only the right or left ventricle. FIGS. 13 and 14 depict therefractory periods associated with right-only and left-only ventricularpacing, respectively. During left-only ventricular pacing, the leftventricular paces are inhibited by either left ventricular or rightventricular sensed events. FIG. 15 depicts the refractory periods aftera right ventricular safety pace delivered during left ventricle-onlypacing when the left ventricle pace is inhibited by an LVPP. Duringright-only ventricular pacing, right ventricular paces are inhibited byonly right ventricular sensed events. Since left ventricular sensing maystill be used for purposes other than pace inhibition, such asdiagnostic counting of left ventricular senses, implementation of arefractory period for the left ventricular sensing channel may benecessary while in a right-only ventricular pacing mode. As shown inFIGS. 13, 14, and 15, all refractory periods ARPVP, RVRPVP, and LVRPVPstart at the instant of the right ventricular pace in the case ofright-only ventricular pacing, and at the instant of the leftventricular pace in the case of left-only ventricular pacing.

[0063]FIGS. 16, 17, and 18 show the refractory intervals of the sensingchannels when the pacemaker is operated in biventricular pacing modewith ventricular triggering. In this mode, rather than inhibiting pacingupon receipt of a right ventricular sense, ventricular pacing istriggered to occur in the shortest time possible after the sense inorder to produce a coordinated contraction of the ventricles. This modeof pacing may be desirable when the inherent intraventricular conductiontime of the heart is long enough that the pacemaker is able to reliablyinsert a pace before depolarization from the right ventricle would reachthe left ventricle. The time delay between a right ventricular sense andthe ensuing pace output is dictated by the response time of the hardwareand is designated in FIGS. 16, 17, and 18 by the sense to pace latencyinterval SPL. The mode may operate such that following a rightventricular sense, either the left ventricle only is paced as shown inFIGS. 16 and 18, or both ventricles are paced as shown in FIG. 17. Inthe latter case, the right ventricle is paced even though a rightventricular sense has been received to allow for the possibility thatthe right ventricular sense was actually a far-field left ventricularsense in the right ventricular channel. If the right ventricular sensewere actually from the right ventricle, the right ventricular pace wouldoccur during the right ventricle's physiological refractory period andbe of no consequence. With either type of ventricular triggeredbiventricular pacing mode, pacing of the left ventricle is inhibited ifit would be delivered during the LVPP interval. FIGS. 16, 17, and 18show that in ventricular triggered mode, separately selectablerefractory period intervals ARPVT, RVRPVT, and LVRPVT for the atrial andventricular sensing channels are triggered by either a univentricular orbiventricular pace. FIGS. 16, 17, and 18 also show that the rightventricle and atrium are refractory during the SPL. The atrial and rightventricular refractory periods during the SPL (ARPRVS, RVRPRVS) aretriggered by the right ventricular sense and are truncated at the timeof the ventricular pace.

[0064] Although the invention has been described in conjunction with theforegoing specific embodiment, many alternatives, variations, andmodifications will be apparent to those of ordinary skill in the art.Such alternatives, variations, and modifications are intended to fallwithin the scope of the following appended claims.

What is claimed is:
 1. A method for operating a cardiac rhythmmanagement device, comprising: sensing a rate chamber and a synchronizedchamber through separate channels and generating sense signals upondetection of depolarization occurring in a chamber; pacing a chamberupon expiration of an escape interval without receipt of a rate chambersense signal from the rate chamber sensing channel; rendering eachsensing channel refractory for a separately selected pacing refractoryperiod after a pacing event; and, rendering a sensing channel refractoryfor a selected sensing refractory period after receipt of a sense signalfrom that channel while continuing to sense in the other channel.
 2. Themethod of claim 1 wherein the rate and synchronized chambers are thepaired ventricles, designated as the ventricular rate chamber and theventricular synchronized chamber.
 3. The method of claim 1 wherein therate and synchronized chambers are the paired atria.
 4. The method ofclaim 1 wherein the rate and synchronized chambers are sensing/pacingsites in the same heart chamber.
 5. The method of claim 1 furthercomprising sensing a plurality of synchronized sensing/pacing sites inthe synchronized chamber through separate channels and generating sensesignals upon detection of depolarization occurring at each site.
 6. Themethod of claim 5 further comprising pacing a plurality of synchronizedsensing/pacing sites in the synchronized chamber through separatechannels.
 7. The method of claim 1 wherein the paced chamber is the ratechamber.
 8. The method of claim 1 wherein the paced chamber is thesynchronized chamber.
 9. The method of claim 1 further comprising pacingboth chambers upon expiration of an escape interval without receipt of arate chamber sense signal from the rate chamber sensing channel.
 10. Themethod of claim 9 wherein the chambers are paced with a selectedsynchronized chamber channel offset interval.
 11. The method of claim 10wherein the pacing refractory period of each chamber begins with pacingof the earlier paced chamber.
 12. The method of claim 11 wherein aminimum refractory period is maintained in the later paced chamber afterthe later pace.
 13. The method of claim 1 wherein a sense in one chambertriggers a pace in the other chamber.
 14. The method of claim 13 whereinthe pacing refractory period of both chambers begins when the pacedchamber receives the pace.
 15. The method of claim 2 further comprising:sensing an atrium through a separate atrial channel and generating anatrial sense signal upon detection of depolarization occurring in theatrium; pacing a ventricle upon expiration of a selectedatrioventricular time interval without receipt of a ventricular ratechamber sense signal from the ventricular rate chamber sensing channel,where the atrioventricular time interval starts with receipt of anatrial sense signal; rendering the atrial sensing channel refractory fora selected p acing refractory period after a ventricular pacing event;and, rendering the atrial sensing channel refractory for a selectedsensing refractory period after a ventricular rate chamber sensingsignal but continuing to sense in the atrial channel following aventricular synchronized chamber sensing signal.
 16. The method of claim15 further comprising: pacing an atrium upon expiration of a selectedatrial escape interval without receipt of an atrial sense signal, wherethe atrial escape interval starts with either receipt of a ventricularrate chamber sense signal or a ventricular pacing event; and, renderingthe atrial and both ventricular sensing channels refractory forseparately selected periods of time after an atrial pacing event. 17.The method of claim 15 further comprising rendering the atrial sensingchannel refractory for a selected period of time after an atrial sense.18. The method of claim 15 further comprising rendering the ventricularsynchronized chamber sensing channel refractory for a selected period oftime after an atrial sense.
 19. A cardiac rhythm management device,comprising: separate sensing channels for sensing a rate chamber and asynchronized chamber and generating sense signals upon detection ofdepolarization occurring in a chamber; a pacing channel for deliveringpacing pulses to a chamber upon expiration of an escape interval withoutreceipt of a rate chamber sense signal from the rate chamber sensingchannel; a controller for controlling the delivery of paces inaccordance with a programmed pacing mode; a pacing refractory periodtimer for rendering each sensing channel refractory for a selectedperiod of time after a pacing event; and, a sensing refractory periodtimer for rendering the rate chamber sensing channel refractory for aselected period of time after receipt of a rate chamber sense signalwithout affecting the synchronized chamber sensing channel.
 20. Thedevice of claim 19 wherein the rate and synchronized chambers are thepaired ventricles, designated as the ventricular rate chamber and theventricular synchronized chamber.
 21. The device of claim 19 wherein therate and synchronized chambers are the paired atria.
 22. The device ofclaim 19 wherein the rate and synchronized chambers are sensing/pacingsites in the same heart chamber.
 23. The device of claim 19 furthercomprising separate channels for sensing a plurality of synchronizedsensing/pacing sites in the synchronized chamber and generating sensesignals upon detection of depolarization occurring at each site.
 24. Thedevice of claim 19 further comprising separate channels for pacing aplurality of synchronized sensing/pacing sites in the synchronizedchamber.
 25. The device of claim 19 further comprising a second pacingchannel for delivering paces to both chambers.
 26. The device of claim25 wherein the controller is programmed to pace one chamber uponexpiration an escape interval and pace the other chamber within aselected synchronized chamber offset interval.
 27. The device of claim26 wherein the pacing refractory period timer for each chamber is resetupon pacing of the earlier paced chamber.
 28. The device of claim 25wherein the controller is programmed to pace the synchronized chamberwithin a latency period after receipt of a rate chamber sense signal.29. The device of claim 25 wherein the controller is programmed to paceboth chambers within a latency period after receipt of a rate chambersense signal.
 30. The device of claim 28 wherein the pacing refractoryperiod timers for both chambers are reset after the pace is delivered.31. The device of claim 26 wherein the pacing refractory period timersfor both ventricles are reset upon pacing of the earlier pacedventricle.
 32. The device of claim 20 further comprising: an atrialsensing channel for sensing atrial depolarizations and generating atrialsense signals in accordance therewith; an atrioventricular timer fortriggering a ventricular pace upon expiration of a selectedatrioventricular time interval without receipt of a ventricular ratechamber sense signal from the ventricular rate chamber sensing channel,where the atrioventricular timer is reset upon receipt of an atrialsense signal; a refractory period timer for rendering the atrial sensingchannel refractory for a selected pacing refractory period after aventricular pacing event; and, a refractory period timer and logiccircuitry for rendering the atrial sensing channel refractory for aselected sensing refractory period after a ventricular rate chambersensing signal, wherein the atrial sensing channel continues to sensefollowing a ventricular synchronized chamber sensing signal.
 33. Thedevice of claim 32 further comprising: an atrial pacing channel fordelivering paces to an atrium; wherein the controller is programmed todeliver paces to the atrium in accordance with an atrial pacing mode;and, pacing refractory period timers for rendering the atrial and bothventricular sensing channels refractory for selected refractory periodsafter an atrial pacing event.